fix: symlink 探测有界并发 + UNC 标记平台中性化（回应 CR/自审）

- fs-tree: symlink 目标 stat 从串行改为 32 并发工作池（readdir 流关闭后统一
  探测），symlink 密集/网络目标目录不再累加延迟；顺序与分类语义不变
- html-route: 解码不再重建反斜杠 UNC，统一重建为平台中性的 //server/share/...
  前斜杠形态（win32/POSIX 均正确 resolve），从而删除 windowsPathStyle 平台信号
  与 client isWindowsStylePath 助手；同时解决无 cwd 时正斜杠 UNC 的往返盲点
- 测试/文档同步；移除未使用的 isPosix

src/html-route.ts

@@ -13,16 +13,17 @@
  *   /sidebar/html/S/Users/me/proj/index.html
  *     + ./style.css → /sidebar/html/S/Users/me/proj/style.css
  *   Windows: C:\Users\me\a.html → /sidebar/html/S/C%3A/Users/me/a.html
- *   Windows UNC (windowsPathStyle): \\server\share\proj\a.html
+ *   UNC (\\server\share\... or //server/share/...):
  *     → /sidebar/html/S//server/share/proj/a.html  ('//' right after the
  *       sessionId marks the UNC prefix; the WHATWG URL keeps '//' intact so
  *       relative assets still resolve inside the same route)
  *
- * The `windowsPathStyle` flag is the platform guard: a leading `//` is a
- * legitimate POSIX absolute path, so the UNC marker is only emitted for
- * sessions whose cwd shape identifies them as Windows (see the client's
- * isWindowsStylePath) — otherwise a POSIX `//server/share/...` would decode
- * back to a backslash UNC form the host then rejects.
+ * The decoder rebuilds the marker as a forward-slash `//server/share/...`
+ * path. That form is intentionally platform-neutral: `node:path` resolves it
+ * to `\\server\share\...` on win32 and `/server/share/...` on POSIX, so the
+ * host's existing requireAbsolute + isWithin fence needs no platform signal
+ * (a leading `//` is a legal POSIX absolute path, so no data is lost on
+ * either platform).
  *
  * This module is intentionally dependency-free (no node imports, no wire
  * helpers) so the client bundle can import `encodeHtmlUrl` without tripping
@@ -45,15 +46,9 @@ export type HtmlDecodeResult =
 /** The route prefix both encoders/decoders agree on. */
 export const HTML_ROUTE_PREFIX = '/sidebar/html/'
 
-/**
- * Build the route URL for one absolute file path (client + tests).
- * @param windowsPathStyle - whether the path belongs to a Windows-style
- * session (drive or UNC). Only then is a `\\`/`//`-prefixed UNC path marked
- * with the '//' marker; POSIX sessions keep `//` as an ordinary absolute
- * path (see the module comment for the platform guard rationale).
- */
-export function encodeHtmlUrl(sessionId: string, path: string, windowsPathStyle = false): string {
-  const unc = windowsPathStyle && /^[\\/]{2}[^\\/]/.test(path)
+/** Build the route URL for one absolute file path (client + tests). */
+export function encodeHtmlUrl(sessionId: string, path: string): string {
+  const unc = /^[\\/]{2}[^\\/]/.test(path)
   const segments = path.split(/[\\/]+/).filter(segment => segment !== '')
   return `${HTML_ROUTE_PREFIX}${encodeURIComponent(sessionId)}/${unc ? '/' : ''}${segments.map(encodeURIComponent).join('/')}`
 }
@@ -84,9 +79,9 @@ export function decodeHtmlUrl(pathname: string): HtmlDecodeResult {
     return { ok: false, status: 400, message: 'sessionId and file path are required' }
   }
   // An empty FIRST path segment is the UNC marker (encodeHtmlUrl emits
-  // '<sid>//server/share/...' for Windows UNC paths); the encoder filters
-  // empty segments everywhere else, so an empty segment can only be the
-  // marker or a malformed URL — both handled here.
+  // '<sid>//server/share/...' for UNC paths); the encoder filters empty
+  // segments everywhere else, so an empty segment can only be the marker or
+  // a malformed URL — both handled here.
   const unc = pathSegments[0] === ''
   const tail = unc ? pathSegments.slice(1) : pathSegments
   if (tail.length === 0 || tail.some(segment => segment === '')) {
@@ -94,9 +89,9 @@ export function decodeHtmlUrl(pathname: string): HtmlDecodeResult {
   }
   let path: string
   if (unc) {
-    // Rebuild the UNC form \\server\share\... so requireAbsolute() +
-    // isWithin(cwd) see the same path the explorer works with.
-    path = `\\\\${tail.join('\\')}`
+    // Rebuild the platform-neutral forward-slash form `//server/share/...`;
+    // requireAbsolute() resolves it to the platform's own UNC/POSIX spelling.
+    path = `//${tail.join('/')}`
   } else if (/^[A-Za-z]:$/.test(tail[0] ?? '')) {
     // A Windows drive segment ('D:') is the FIRST path segment of an encoded
     // drive path. Rejoining it with a leading slash would yield '/D:/work/...'